docs(material/button-toggle): add description for appearance property in default options (#22852)

* docs(material/button-toggle): add description for appearance property in default options

Developers can specify a default appearance for button toggles
using the `MatButtonToggleDefaultOptions`. The appearance property
within these default options is not documented. This commit adds
a description.

* fixup! docs(material/button-toggle): add description for appearance property in default options

Address feedback

(cherry picked from commit c65a87b)

Author: devversion

src/material/button-toggle/button-toggle.ts

@@ -54,6 +54,10 @@ export type MatButtonToggleAppearance = 'legacy' | 'standard';
  * using the `MAT_BUTTON_TOGGLE_DEFAULT_OPTIONS` injection token.
  */
 export interface MatButtonToggleDefaultOptions {
+  /**
+   * Default appearance to be used by button toggles. Can be overridden by explicitly
+   * setting an appearance on a button toggle or group.
+   */
   appearance?: MatButtonToggleAppearance;
 }